As a method for suppressing knocking of an internal combustion engine, it is know to use an EGR (exhaust gas recirculation) device that mixes a part of the exhaust gas of an internal combustion engine with intake air as EGR gas, and forwards the mixture to the intake system of the internal combustion engine. Since knocking diminishes as the temperature of the gas fed to the intake system decreases, it is desirable to lower the temperature of the gas fed to the intake system as much as possible. Since the exhaust gas of the internal combustion engine is high in temperature, the EGR apparatus for performing such an EGR operation is often provided with an EGR cooler for cooling the exhaust gas.
When the EGR gas is cooled by an EGR cooler using the atmospheric air as the cooling medium, the EGR gas may not be adequately cooled when the temperature of the available air is excessively high. For this reason, it has been proposed to use a cooling circuit utilizing the thermal energy of the exhaust gas to cool the compressed intake air and/or the EGR gas fed to the internal combustion engine to a desired level. See JP2013-527369A, for instance.
According to this prior art, the cooling circuit includes a vapor generator (first evaporator), an evaporator (second evaporator), and an ejector pump. When the high temperature exhaust gas discharged from the internal combustion engine is passed through the first evaporator, the coolant stored in the first evaporator turns into high pressure vapor. By introducing the high pressure coolant into the ejector pump, the second evaporator is depressurized, and the internal temperature of the second evaporator is lowered. As the temperature of the second evaporator decreases, the compressed intake air and/or the EGR gas flowing through the interior of the second evaporator is cooled.
Also, when the temperature of the EGR gas is higher than the temperature of the intake air, the moisture contained in the EGR gas may condense as soon as the EGR gas is mixed with the intake air. Since the water droplets are harmful to the turbocharger of the internal combustion engine, it is necessary to cool the EGR gas to a temperature level lower than that of the intake air before mixing the EGR gas with the intake air. JP2013-527369A also discloses the provision of a second evaporator in a passage for feeding the EGR gas so that only the EGR gas is cooled.
JP2013-527369A describes a certain cooling circuit as a first embodiment, in which exhaust gas minus the EGR gas is passed through a first evaporator and the EGR gas is passed through a second evaporator. In this cooling circuit, since the EGR gas does not pass through the first evaporator, and flows directly into the second evaporator while still in a high temperature condition, there is a possibility that the EGR gas may not be sufficiently cooled when introduced into the second evaporator. Furthermore, JP2013-527369A discloses a slightly different cooling circuit as a second embodiment, in which the EGR gas is allowed to pass through a first evaporator, and the EGR gas that has passed through the first evaporator is then passed through a second evaporator. In this cooling circuit, since the gas passing through the first evaporator consists solely of the EGR gas, the thermal energy obtained in the first evaporator may not be sufficient for cooling the EGR gas passing through the second evaporator.